cli: unbreak dulwich script

The first value in sys.argv is the name of the script invoked,
with actual arguments following. Previously, any attempt to invoke the
script would yield e.g.:

$ dulwich
No such subcommand: /path/to/dulwich

This was presumably never noticed due to python -m dulwich.cli
explicitly passing in sys.argv[1:].

See https://docs.python.org/3/library/sys.html#sys.argv for details.